For oil it's a 10W40 grade. If you leave the oil in there for 12,000 miles, then I'd use synthetic or a semi-synthetic that has high detergent properties (an oil labelled "for diesel engines"... or something like Motul 6100.. which has an ACEA A4 rating).
If you change the oil at 6,000 miles then any good-brand semi-synthetic 10W40 will do. Fuchs is not bad.
I find on mine that leaving the oil in there for 12k intervals causes a bit of gum and varnish to build up, though it's hardly serious. Synthetic has a higher detergent content than semi-synthetic so keeps the beast clean for longer.
Oil filter - just use the OE job since it's hard to improve on it and you don't want it collapsing internally if it's just a cheapo one. The filter lives at the back of the engine, next to the alternator. You need to borrow your neighbour's 8-year old or buy one of those organ monkeys to go in there and change it.
If you jack the car up (and support it) then you can get access to it from below.. once you got your arm round the steering, suspension, sub-frame and drive-shafts.
For plugs there's 2 schools of thought: Leave them in there forever (fit Iridium) or change them every year (fit regular). I use Champion Eon and I never had any spark-plug issues. We're on about 20k now. But do take the plugs out once a year to wire-brush clean and then lightly copper-grease the threads or they'll seize in there.
For air filter.. use the standard paper filter. Panel filters habitually wreck the MAF sensor etc. if you get oil and gunge from the air-filter on the sensitive sensory components. Paper OE filter on mine gets thrown out once a year but everything downwind of the airbox is 19 years old and 220,000 miles originally out of the factory.
Oil light flashing could be oil surge making the oil end up away from the level indicator.. or you could have low oil. Fill the beast to MAX on the dip-stick and always keep the level there.
The temp' sensor is on the thermostat so may have been covered in grime and the wiring become a bit corroded or broken off. The radiator fan switch is on the radiatore so that's a separate circuit.